Although I have them all, and multiple recordings of many, they're just so consistently good, and so bloody numerous, that maybe I still don't have any overall favourites, or rather: the ones I could name as my favourites are the ones I spent the most time with--not necessarily because they're my favourites. And yes I know how convoluted that sounds. And sometimes I'm thinking, 'what number was that one with...?'

That said, 51, a solo cantata for soprano, springs to mind.

Saying cantata 191 is cheating, but it does have some of the most beautiful music he ever wrote.

You mentioning BWV 51 reminds me of a real cheat, BWV170 A setting of Psalm 51 which ingeniously and beautifully uses Pergolesi's music to his famous version of the Stabat Mater. Not a top five of Bach's pieces, but well worth seeking out and admiring how well Bach applies an alternative text.

Amongst my top five for Bach: BWV170 Vergnuegte Ruh. This is one of his very few solo santatas, usually performed in the alto voice version. The lilting opening movement has one of Bach's most memorable, long, sinuous melodies.

There are a number of very fine recordings: Janet Baker, Scholl, Fink, Jaroussky etc. The older ones are not necessarily slower than the modern versions. I am on Spotify listening to Scherchen driving Rossel-Madjan with a small body of players from 1952. He really pulls out the dance aspect of the rhythms of the first movement, rather overpowering the singer. But it is ear opening.